Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver salt photography (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)” published by Toray Research Center Inc., 1993, pp. 241-285; and “Printer Zairyo no Kaihatsu (Development of Printer Materials)” published by CMC Publishing Co., Ltd., 1995, p. 180). Moreover, this system has advantages over silver salt photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
In an image-receiving sheet on this system, a receptor layer for fixation of dyes transferred to the sheet is formed on a support, and besides, a layer having high cushion properties, such as a foam layer made up of a resin and a foaming agent or a porous layer containing hollow polymer particles, is generally formed between the support and the receptor layer with the intention of enhancing adhesion of the image-receiving sheet to a transfer sheet (see, e.g., JP-A-11-321128 (“JP-A” means unexamined published Japanese patent application) and JP-A-2-89690).
The intermediate layer containing hollow polymer particles is known to be an effective medium for ensuring a high transfer density because of its heat insulating effect. On the other hand, the use of such an intermediate layer as to contain hollow polymer particles causes a problem of aggravating smoothness of the resulting heat-sensitive transfer image-receiving sheet to result in an increase of defects of transferred dye.
When a symbolic, textual or numeric image is transferred, defects of transferred dye are generally inconspicuous because the proportion of white portions is high. In the cases of pictorial images and images including flatly colored portions, however, defects of transferred dye are conspicuous and become a serious problem.
JP-A-11-321128 discloses the formation of a receptor layer by using a coating solution of resin in an organic solvent after an intermediate layer containing as main components hollow particles and a polymer resistant to an organic solvent is formed on a support by coating and drying. In JP-A-11-321128, however, the receptor layer is not specified, so there arises problems, such as defects of transferred dye.